Calculating total time of multiple repeating sections

[Dark Factor]

I have a form that uses repeating sections, each of which has a time duration
that the user sets. I am trying to calculate the total time of all sections
as they are added, removed, and edited.

The sum function works for integers, but doesn't seem to work for time. I am
not versed in xml, though I suspect there would be a way to do the
calculations this way. Any suggestions for approaching this?

thanks